Examples of steel defect detection techniques include an ultrasonic testing method, a magnetic flux leakage inspection method, a magnetic particle inspection method, an eddy current inspection method, and an optical method.
In the magnetic flux leakage inspection method, magnetic flux leakage from surfaces of a steel plate is measured using a magnetic sensor such as a Hall element capable of converting magnetic flux into an electric signal, and it is determined whether the steel plate has defects based on the measured magnetic flux leakage.
FIG. 1 illustrates an apparatus for detecting defects of a steel plate by the magnetic flux leakage inspection method.
Referring to FIG. 1, the steel plate defect detection apparatus may include: a magnetizing unit 110 generating magnetic flux in a direction of transfer of a steel plate S to magnetize the steel plate S; a magnetic sensor array 120 arranged along the width of the steel plate S for measuring leakage magnetic flux when magnetic flux passes through defects of the steel plate S; and an amplifying unit 130 amplifying a signal output from the magnetic sensor array 120 according to leakage magnetic flux. The magnetizing unit 110 may include a permanent magnet PM and yokes 111a and 111b extending from both sides of the permanent magnet PM.
However, when defects of the steel sheet S are detected using the steel plate defect detection apparatus, since the steel plate S is magnetized in a direction of transfer of the steel plate S, leakage magnetic flux measured using the magnetic sensor array 120 contains information about defects existing on the surfaces of the steel plate S as well as defects existing inside the steel plate S. That is, it is difficult or impossible to detect only defects existing inside the steel plate S.